The present invention relates to an improvement in a non-aqueous electrolyte comprising a solvent and a solute dissolved in the solvent. The present invention also relates to a non-aqueous electrolyte battery which contains an improved non-aqueous electrolyte, and which is excellent in the storage property and in the charge/discharge cycle characteristics.
In recent years, there has been advancement in the realization of small and lightweight electronic devices, and along with that, there has also been an increased demand for batteries having high energy densities. Accordingly, intensive researches have been made on lithium primary batteries having a negative electrode comprising metallic lithium as well as lithium ion secondary batteries having a negative electrode comprising a carbon material.
In such batteries, as a solvent for constituting a non-aqueous electrolyte, propylene carbonate, ethylene carbonate, butylene carbonate, sulfolane, xcex3-butyrolactone, dimethyl carbonate, diethyl carbonate, 1,2-dimethoxyethane, tetrahydrofuran, dioxolane and the like are used singly or as a mixture. Further, as a solute to, be dissolved in the solvents, LiClO4, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2 are used singly or as a mixture.
Recently, intensive researches have been made on lithium polymer batteries containing a gel non-aqueous electrolyte or a solid polymer electrolyte. The gel non-aqueous electrolyte contains a host polymer for retaining the solute and the solvent as described above. The solid polymer electrolyte is an electrolyte in which the polymer itself functions as the solvent for the solute, and a polymer similar to the host polymer contained in the gel-like non-aqueous electrolyte is used, for example.
As the polymers constituting these electrolytes, derivatives formed on the basis of polyvinylidene fluoride, polyethylene oxide, polyacrylonitrile, polymethyl methacrylate, polysiloxane and the like are used.
These constituting elements, of the non-aqueous electrolyte are known to chemically, react with moisture and the electrodes inside the battery. In particular, metal lithium, lithium alloys (LiAl, LiSn and the like) and carbon materials capable of absorbing and desorbing lithium, which constitute the negative electrode, are highly reactive with the constituting elements of the non-aqueous electrolyte, and they form an organic coating film on the surface of the negative electrode by chemical reactions and the like, thereby to decrease the battery capacity.
In a primary battery as well as in a secondary battery, the longer the storage period of the battery and the higher the storage temperatures, the more obvious the influence of the chemical reactions becomes and the more significantly the capacity deteriorates. Also, there is the problem that repetition of the charge/discharge cycle of the secondary battery causes capacity deterioration.
In order to suppress the reaction between the negative electrode and the non-aqueous electrolyte during a storage, there has been proposed in Japanese Laid-Open Patent Publication No. 6-215775 to make the non-aqueous electrolyte contain a salt comprising a specific organic anion and a metal cation. However, even in the battery containing the non-aqueous electrolyte proposed in the above publication, deterioration in the battery capacity is observed during a long-term storage at high temperatures; in secondary batteries, a decrease in the discharge capacity is observed at the time of repetition of the charge/discharge cycle.
In the present invention, used is a non-aqueous electrolyte which is not liable to react with the negative electrode in a primary battery or a secondary battery during a long-term storage at high temperatures, and which does not lead to a decrease in the capacity during the charge/discharge cycles in a secondary battery.
The present invention relates to a non-aqueous electrolyte comprising a solvent and a solute dissolved in the solvent, wherein the solute comprises at least one selected from the group consisting of lithium salts represented by the general formula (1): 
where X1 to X4 are independently hydrogen atoms, F, Cl, Br, I or alkyl groups having 1 to 3 carbon atoms, and lithium salts represented by the general formula (2): 
where X5 to X8 are independently hydrogen atoms, F, Cl, Br, I or alkyl groups having 1 to 3 carbon atoms.
The aforementioned solute may further comprise LiPF6, LiBF4, LiClO4, LiCF3SO3 and lithium salts having an imide bond represented by the general formula (3):
xe2x80x83LiN(CmX92m+1SO2)(CnX102n+1SO2)
where X9 and X10 are independently F, Cl, Br or I, and m and n are independently integers of 1 to 4. Among these, lithium salts having an imide bond represented by the general formula (3) are particularly preferred.
Specific examples of lithium salts having an imide bond represented by the general formula (3) include LiN(CF3SO2)2, LiN(C2F5SO2)2 and LiN(CF3SO2)(C4F9SO2). Among these, LiN(CF3SO2)2 and/or LiN(C2F5SO2)2 are particularly preferred.
As the aforementioned solvent, an organic solvent is preferred, and it preferably comprises at least one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate and y-butyrolactone.
The concentration of the lithium salt represented by the general formula (1) and/or the lithium salt represented by the general formula (2) is preferably 0.2 to 2.0 mol/liter from the viewpoint of the solubility of the lithium salt and the electric resistance. Also, in the case where the aforementioned solute comprises the lithium salt represented by the general formula (1) and/or the lithium salt represented by the general formula (2) together with another lithium salt, the concentration of the total of the lithium salts are preferably 0.2 to 2.0 mol/liter from the same viewpoint.
The present invention also relates to a non-aqueous electrolyte battery comprising; a positive electrode; a negative electrode comprising at least one selected from the group consisting of metallic lithium, an lithium alloy, and a material capable of absorbing and desorbing lithium; and a non-aqueous electrolyte of the present invention.